Although conventional inorganic semiconductor materials have good properties and reliability, they are being replaced with organic semiconductor materials because organic semiconductor materials realize a simple preparation process, enable the inexpensive manufacture of a device, and facilitate the development of materials exhibiting superior properties through simple structural changes to organic substances.
An example of technical applications for organic semiconductor materials includes organic photovoltaic cells.
The photovoltaic cell is composed fundamentally of a semiconductor layer and electrodes. When external light is incident on the photovoltaic cell, electrons and holes are generated in the semiconductor layer, and these charges are moved toward the corresponding poles P and N, respectively, thus generating a difference in potential between the poles P and N. At this time, a load is connected to the photovoltaic cell so that current flows, thereby producing electricity.
When the organic semiconductor material is applied to the photovoltaic cell, it is advantageous in that the device may be easily manufactured using a solution process, such as spin coating, instead of conventional sputtering in a high vacuum.
In 1995, Fred Wudl's group reported {6}-l-(3-(methoxycarbonyl)propyl)-{5}-l-phenyl[5.6]C61, which is a methanofullerene derivative known as PCBM [J. Org. Chem., 1995, 60, 532].
PCBM may be utilized for an organic photovoltaic cell by blending it with donor polymers, such as MEH-PPV, MDMO-PPV, and P3HT. Initially, PCBM was used in a manner of blending with a PPV derivative at a ratio of about 1:3, in order to manufacture the device. In accordance with recent reports, PCBM is blended with P3HT to manufacture a device, which is then thermally treated at high temperatures, or the evaporation rate of a solvent is controlled upon the production of a thin organic film, resulting in photovoltaic devices having high power conversion efficiency of 4% or more.
However, it is difficult to ensure reproducibility using such post-treatment. In the case where the device is subjected to high temperatures, the morphology of the organic film is changed, adversely affecting the efficiency or other device properties.
Further, typical examples of a process of forming an active layer in an organic photovoltaic device using an organic semiconductor material include spin coating, screen printing, and inkjet printing, enabling the easy formation of layers having large areas.
A research team, led by professor G. E. Jabbour, the university of Arizona, USA, 2001, reported an organic photovoltaic device, which is manufactured through screen printing using a blend of MDMO-PPV:PCBM and exhibits power conversion efficiency of 4.3% under a monochromatic laser at 488 nm of 27 mW/cm2, but is problematic in that the above efficiency is merely obtained under the monochromatic laser, whereas the use of an actual light source of AM 1.5 G results in very low efficiency [Appl. Phys. Lett., 79, 2996 (2001)].
In 2005, an organic photovoltaic device having power conversion efficiency of 1.8˜2.4% was manufactured by Matsushita Co. Ltd., Japan, by screen printing MDMO-PPV:PCBM, [IEEE Photovoltaic Specialists Conference, 31st, 125 (2005)], and also, in 2007, F. C. Krebs' group manufactured a large-area flexible organic photovoltaic cell 655.2 cm2 in area through spin coating of MDMO-PPV and vacuum evaporation of C60, but the power conversion efficiency thereof was 0.0002%, which is evaluated to be very low.
In the journal of Adv. Mater., December, 2007, C. J. Brabec reported an organic photovoltaic cell having power conversion efficiency of 2.9% by forming a thin organic semiconductor film through inkjet printing, but this device is disadvantageous because, in the event of formation of a multilayer structure using inkjet printing, interlayer mixing occurs [Adv. Mater. 19, 3973-3978 (2007)].
In addition, an aerosol jet printing process is mainly applied in a manner such that metal ink is atomized using ultrasonic waves or a carrier gas which is rapidly jetted, printed on a curved substrate, and then sintered using a laser, thus forming highly conductive wires (U.S. Pat. Nos. 7,270,844 and 7,294,366).
In accordance with the conventional solution process, such as spin coating or printing for printing solution-phase ink on a substrate, when an organic film is formed and a layer of solution-phase ink is then formed thereon through dropping, the lower organic film is damaged, thus making it impossible to realize multilayered devices, and further, the resultant devices have properties inferior to multilayered devices manufactured through vacuum evaporation. Therefore, in order to solve the above problems, the present inventors have adopted aerosol jet printing so that ink in a mist form having a size of μm or smaller, formed in an aerosol jet, is jetted onto the surface of a substrate or organic film, thereby realizing a photoactive layer having a multilayer structure while the lower organic film is never damaged, and thus have confirmed an increase in the solar power conversion efficiency of the organic photovoltaic cell including a photoactive layer, manufactured through the method of the present invention, thereby completing the present invention.